This is a very nice plugin, but be warned that it will require "sysadmin" privileges on SQL server to run.

Considering that you will also need to activate TCP/IP remote connection and plain SQL authentication for your nagios user to connect to the SQL database, that could be a major security hole in your infrastructure if you do not properly lock the connections between your monitoring server and your SQL Server.

There is some methods you can use to mitigate the risk, including creating a dedicated database for this monitoring, with a dedicated user, and a dedicated stored procedure, as explained here:
http://www.sqlapprentice.net/?p=108

But I would love to have a plugin who could just perform a basic SQL test without requiring admin privileges to the master system database.